A so-called single-sided multilayer substrate, which does not include a core substrate, may be used as a circuit substrate on which to mount an electronic device such as a semiconductor device. The single-sided multilayer substrate is manufactured by alternately laminating conductive layers and insulated layers on each of two sides of a support substrate and finally removing the support substrate. A process at the initial stage of manufacturing a single-sided multilayer substrate will be simply described below.
In the manufacturing of a single-sided multilayer substrate, electrode pads that are connected to the terminals of an electronic device such as a semiconductor device are first formed on each of two sides of a support substrate. Next, insulating sheets are attached onto the two sides of the support substrate to cover the electrode pads with the insulating sheets. Via holes extending to the electrode pads are then formed on each insulating film by, for example, laser machining or etching. To control the depth of the via hole, the electrode pad is used as a film to stop machining. A metal film is deposited on the surface of the insulating film and the interiors of the via hole, forming a wiring pattern and a via simultaneously. A single-sided multilayer substrate is manufactured by repeating these processes. Japanese Laid-open Patent Publication No. 2000-323613 is an example of related art.
Narrow pitches among electrode pads on circuit boards have been demanded recently to match narrow pitches among the terminals of electronic parts. Since the electrode pad is used as a film to stop machining in via hole depth control, that is, as a surface to stop laser machining or etching, however, it is not possible to make the electrode pad smaller than a demanded via diameter. In addition, laser machining or etching involves little machining error (machining position displacement), so it is desirable that the electrode pad be made larger than the demanded via diameter by an amount equal to the machining error. That is, electrode pads on circuit boards used at present are not preferably reduced to or below a size equal to the via diameter. Accordingly, it has been difficult to narrow a pitch between electrode pads by reducing the size of the electrode pad on a circuit substrate.